38 DR. J. W. EVANS ON A MONCHIQTJITE EROM [Eeb. I9OI, 



4. A MoNCHiQTJiTE from Mount Girnar, Jxjnagarh (Kathiawae). 

 By John William Evans, D.Sc, LL.B., E.G.S. (Read Novem- 

 ber 21st, 1900.) 



[Plate II.] 



Contents. 



Page 



I. Monchiquites and their Isotropic Groundmass 38 



II. Analcime as a Eock-forming Mineral 40 



III. A Rock with a Monchiquite-Matris, from Mount Girnar 41 



IV. Chemical Analyses 46 



V. The Nature of the Isotropic Groundmass 48 



VI. The History of the Eock 49 



VII. Bibliography 52 



Sketch-map of the Central Eidge of Mount Girnar , 42 



I. Monchiquites and their Isotropic Groundmass. 



The term moncliiquite is now recognized as the designation of 

 a rock, consisting mainly of ferromagnesian silicates in an isotropic 

 groundmass, which has approximately the chemical composition 

 and specific gravity of analcime. 



The name was given in 1890 by Hunter & Rosenbusch [10]^ 

 to a rock occurring in narrow dykes, near Cabo Frio in the Serra 

 de Tingua and elsewhere in Brazil, and containing pyroxene, soda- 

 hornblende, mica, and olivine-phenocrysts in a colourless — or, rarely, 

 transparent brown — matrix. This material, which they assumed to 

 be a glass, contains numerous microlites of the porphyritic minerals 

 and, when in a fresh condition, is completely isotropic. It has a 

 specific gravity of 2'31. In a few cases nepheline and, rather more 

 frequently, a felspar with twin lamellation occur. The groundmass 

 is very subject to alteration, natrolite and analcime being formed 

 [20] p. 539 & [24] p. 233. In one instance fluidal structure was 

 noticed to be present. The rock was distinguished from camptonite 

 on account of its glassy base, and received the name of monchi- 

 quite, as L. van Werveke had described a similar rock from the 

 Serra de Monchique in Southern Portugal [7].^ 



Rocks with a groundmass of the same character, but without 

 olivine, have been reported from Bohemia by Boficky [4, 5] and Hibsch 

 [15]. Nepheline [5] p. 176, and leucite [15] p. 99, are sometimes 

 present. J. R. Williams [11] described similar rocks from Arkansas, 

 in which he believed a glassj^ base had once existed but had since 

 become devitrified.^ J. E. Kemp found rocks of this class in the 



^ The numbers in square brackets throughout this paper refer to the 

 Bibliography, § VII, p. 52. 



^ In this case the glasslike groundmass showed greyish-blue interference- 

 colours under crossed nicols, and Avas described as nepheline. 



^ He proposed to separate the varieties without olivine, under the names of 

 fourchite for those containing amphibole and pyroxene, and ouachitite 

 for those containing biotite. But Eosenbusch does not consider olivine an 

 essential constituent [20] p. 545 & [24] p. 233, and in any case the new names 

 appear to be unnecessary. 



